After almost 36 years as a registered Republican, Ms. Margarella, of West Brighton, filed change-of-registration paperwork with the Staten Island Board of Elections on Thursday.

Ms. Margarella said the switch would allow her to be more involved with community issues on the heavily Democratic North Shore.

"There's not much I can do on this side of the Island as a Republican," she said. "I feel more a part of the community as a Democrat."

In March, Ms. Margarella ran as a Republican-Conservative against Democratic-Working Families candidate Matthew Titone in a special election for John Lavelle's seat. She took 1,866 votes, or about 32 percent, to Titone's 2,888 votes (49 percent).

Lavelle, the borough's Democratic Party chairman, died suddenly after sustaining a massive stroke in late January.

In 2006, Ms. Margarella, a 53-year-old former eighth-grade social studies teacher at Markham Intermediate School in West Brighton, ran against Lavelle for his seat, garnering 28 percent of the vote.

For her run this year, Ms. Margarella was selected unanimously by the Staten Island GOP county committee, and also carried the endorsement of former Senator John J. Marchi.

Democrats outnumber Republicans in the North Shore Assembly district almost three to one.

Ms. Margarella said the change had nothing to do with her failed Assembly campaigns or with any dissatisfaction with the Republican Party.